The examples below are from the Mosaickr.com site. You will need to have a flickr account, but you do not need a lot of photos uploaded. You can create these mosaics using open source images others have taken right on flickr (Cleveland, Ohio has over 600,000 images). You can get the Small mosaic for free and it downloads directly to your computer. You can also pay a fee and get much larger images. The downloads are fine for most application in school.
1. Historical figures with images from their life or geography mosaics
2. LA: What book are you reading? Images of that subject
3. Science: Ocean Theme, Weather, Animals, and more
4. Math: Great architecture of skyscrapers or buildings (even my hometown of 45,000 has 29 images of buildings, Newark Ohio)
5. Art: Famous Artist with their work
6. What would you add?